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You have a large rectangular cake, and someone cuts out a smaller rectangular piece from the middle of the cake at a random size, angle and position in the cake (see the picture below). Without knowing the dimensions of either rectangle, using one straight (vertical) cut, how can you cut the cake into two pieces of equal area?

enter image description here

enter image description here

Here is my attempt at the solution. I assume that all the dimensions are known.

Area of large rectangular cake $=ab\text{ }(a,b\in \mathbb{R}^+,a>b)$

Area of smaller rectangular piece $=cd\text{ }(c\in [0,a],d\in [0,b],c>d)$

Total area $=ab-cd$

Area right of vertical cut $=mb$ (we need to determine $m$)

Area left of vertical cut $=(a-m)b-cd$

Equate both expressions: $mb=(a-m)b-cd$

$$cd=(a-2m)b$$

Therefore

$$m=\frac{1}{2}\left(a-\frac{cd}{b}\right)$$

Is there a way to answer the question without assuming we know the dimensions.

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    $\begingroup$ The word "vertical" in the question is confusing. You might change it to, "one straight cut". $\endgroup$
    – daniel
    Oct 8, 2016 at 20:34
  • $\begingroup$ @daniel, if we imagine a 3D picture of a rectangular cake, the problem is trying to say that we should not cut the cake horizontally (half way of its height) because cutting horizontally will also solve the problem and makes two equal parts. Therefore vertical cuts in here means to cut the cake from top surface to its bottom. $\endgroup$
    – Seyed
    Oct 8, 2016 at 22:49
  • $\begingroup$ @Seyed: yes i know what the problem is trying to say. a 'vertical line' with reference to the picture is not what is meant. $\endgroup$
    – daniel
    Oct 9, 2016 at 5:13
  • $\begingroup$ It's probably not necessary to even use the phrase "vertical cut"... Assuming we are dealing with separating the new shape into two equal parts in terms of area, we can just deal with a single "cut" along the provided plane. $\endgroup$
    – Eliseo D'Annunzio
    Feb 13, 2019 at 1:10

1 Answer 1

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You just need to find the center of each rectangle. enter image description here

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  • $\begingroup$ Could you explain why this works. $\endgroup$
    – Hogg
    Oct 8, 2016 at 20:23
  • $\begingroup$ The question specifies a vertical cut. Where do you cut? $\endgroup$
    – daniel
    Oct 8, 2016 at 20:23
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    $\begingroup$ By vertical cut mean cut the cake from top surface of the cake to the bottom vertically. $\endgroup$
    – Seyed
    Oct 8, 2016 at 20:26
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    $\begingroup$ @hogg, any line through the center of a rectangle cuts it in to two equal halves, and that is true for both the cake and its hole. $\endgroup$
    – Seyed
    Oct 8, 2016 at 20:32
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    $\begingroup$ @hogg, I explained above about the vertical cut. Vertical cut in here means cut the top surface of the cake vertically to its bottom. $\endgroup$
    – Seyed
    Oct 8, 2016 at 20:35

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